Elucidation of mechanisms for stick slip melt fracture of polymer melt (Experimental analysis of unstable flow of high density polyethylene melt)

During polymer melt extrusion from a circular nozzle, the extrudate at low flow rate shows a stable linear shape with smooth surface but the extrudate at higher flow rate shows unstable shapes with spiral, rosary, or zig-zag structures. This phenomenon is known as melt fracture and hampers high throughput production in polymer processing. For linear polymers, synchronized oscillation of flow volume, extrusion pressures, and shape distortion is frequently observed, and it is known as stick-slip melt fracture (SSMF). Since this phenomenon is an oscillation phenomenon, many studies have been performed to analyze the mechanisms based on the oscillation theory. However, most of previous studies have mainly used mathematical calculation, and therefore experimental evaluations of SSMF have rarely been performed. In this study, we conducted an experimental approach to elucidate the mechanism of SSMF by using high density polyethylene as a polymer with varied lengths of the extrusion nozzle. Experimental results were analyzed by the simple model based on the oscillation theory. In the model, we introduced a supply curve representing a relationship between flow rate and pressure at the supplying side and a flow curve representing a flow resistance at the nozzle side. As a result, it was clarified that SSMF is a phenomenon of self-exited oscillation and also relaxation oscillation caused by the compressive elasticity of polymer melt and the negative damping appearing in a flow curve. We verified that SSMF does not occur when the negative damping is lost by repressing slip at inner side of the nozzle. Furthermore, we identified conditions under which SSMF does not occur despite the existence of negative damping; occurrence of SSMF depends on the mode of polymer supplying, a constant flow mode or a constant pressure mode. The former causes SSMF but the latter does not.